Rust Rampage?

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In just two days last fall, Asian soybean rust traveled 500 miles. It was the second time that it's traveled in such an explosive fashion, says Don Hershman, University of Kentucky plant pathologist. “This means that northern growers may not have much notice before rust arrives there.”

The number of counties with rust doubled from 2005 to 2006, many of them in the Ohio and Mississippi River valleys. Losses remained low due to its arrival after harvest, timely scouting and spraying.

“Last November rust spores traveled from Louisiana to Indiana,” says Greg Shaner, professor of botany and plant pathology and Extension specialist at Purdue University. “There had been no evidence that it was creeping north, and then in two days it hit all over the Mississippi River Valley.”

Rust was verified on Iowa soybean tissue recovered from a bin of 2006 Iowa soybeans. The finding drives home that rust spores did indeed land in Iowa, but does not mean that Iowa will have Asian soybean rust in the 2007 soybean crop. “This single case will have no impact at all on the 2007 growing season,” says Palle Pedersen, Iowa State University (ISU) Extension soybean agronomist.

Since Asian soybean rust first struck the U.S. in November 2004, plant pathologists have attacked this exotic disease with a vengeance, monitoring 753 sentinel plots and overwintering sites.

The United Soybean Board (USB) and soybean checkoff have partnered with the North Central Soybean Research Program, as well as government agencies, to again fund soybean sentinel plots across the country in 2007.

“I tell my audiences that the sentinel plot program is their biggest return on their checkoff investment,” says Anne Dorrance, associate professor of plant pathology, Ohio State University.

“The rapid spread of rust spores these last two years taught us that we particularly need to watch the Mississippi River Valley as a conduit to the Midwest,” Shaner says. “And we learned that it may not take a whole lot of rust in the South to provide sufficient inoculum to cause disease in the North. Of great concern to us this spring will be how early rust is found in Mississippi, Louisiana and Texas. “These areas are the most likely source of spores for the soybean belt.

“We've had so little experience with rust that we don't have a handle on what our risk is, one year out of five, 10 or whatever,” Shaner adds.

“We've been better prepared for rust than any other major plant disease, and the Internet gets the information out. Because rust in the Corn Belt does not survive the winter, every year is totally new, depending upon what's going on in the South and what happens locally,” Shaner says.

Unfortunately, rust does survive the winter in the South, much of it on kudzu, a wild legume. The percentage of kudzu with rust has more than tripled from 11% in 2005 to 40% in 2006, says Jim Marois, University of Florida professor of plant pathology. “Rust almost always overwinters south of Tampa,” he says.

Scientists suspect that up to 11 additional legumes might also be vulnerable to rust and therefore spread the disease, says Glen Hartman, plant pathologist at the University of Illinois and the USDA-ARS National Soybean Rust Research Center. We call it Asian soybean rust, but it's actually a legume rust.

As of late February, “Kudzu in south- central Florida was telling us it's been darned cold,” says Carrie Lapaire Harmon, University of Florida Extension plant pathologist and assistant director of the Southern Plant Diagnostic Network. The prospect of fewer spores overwintering is good news, but can also provide a false sense of security, given the disease's power to knock yields by up to 50%, as demonstrated in South America.